Not applicable.
The field of the invention is Hall effect current sensors and more specifically methods and apparatus for mounting a magnetic field sensor within a gap formed by a core or flux guide that surrounds a conductor.
When current passes through a conductor, the current generates a magnetic field including flux that encircles the conductor and that is directed along flux lines in a direction consistent with the well known right hand rule. The field strength is strongest at locations in close proximity to the conductor. The magnitude of current passing through the conductor is directly proportional to the total strength of the magnetic field generated thereby. Thus, if the magnetic flux generated by the current can be accurately determined, then the magnitude of the current passing through that conductor can also be determined.
One way to determine the magnetic flux and hence conductor current has been to design a sensor configuration that relies upon the well known Hall effect electromagnetic principle. To this end, in 1879, Edwin Hall discovered that equal-potential lines in a current carrying conductor are skewed when put in the presence of a magnetic field. This effect was observed as a voltage (Hall voltage) perpendicular to the direction of current flow. Today, Hall effect devices for measuring the Hall voltage and hence a corresponding magnetic field are packaged as single Hall effect chips and are sold as high volume commodity items.
A typical current sensor utilizing Hall effect technology consists of a toroid or rectangular shaped gapped core and a Hall effect chip. Exemplary cores typically include either a laminated stack or a high resistivity solid ferrite material designed to prevent unwanted eddy currents. A single current carrying conductor is positioned within the core such that the permeable core directs the magnetic flux through the core and across the gap. A Hall effect chip is placed within the gap to sense the flux density passing there across. In a well-designed Hall effect current sensor, the measured flux density is linear and directly proportional to the current flowing through the current carrying conductor.
One design challenge routinely faced when designing Hall effect sensors has been finding a cost effective and mechanically robust way in which to mount the Hall effect chip within a core gap. One other challenge has been to configure a sensor that has a relatively small volume footprint. With respect to cost, as with most mechanical products, minimal piece count, less and simplified manufacturing steps and less manufacturing time are all advantageous. In addition, smaller components size typically translates into reduced costs. With respect to robustness, many Hall effect sensors are designed to be employed in rugged environments such as industrial control applications where shock and vibration are routine.
The industry has devised several Hall effect sensor configurations. For instance, in one configuration, a donut shaped and gapped ferrite core is positioned over a vertically mounted Hall effect chip which is soldered to a circuit board. In this case the ferrite core is typically manually positioned with respect to the chip and is then glued to the circuit board. While this solution can be used to provide a robust sensor configuration, this solution has several shortcomings. First, sensor manufacturing experience has revealed that it is relatively difficult to accurately position and glue a donut shaped core relative to the circuit board mounted Hall effect chip. Also, in this regard, where the sensor is subjected to vibrations and shock, any loosening or shifting of the bond between the core and board can compromise the accuracy of the current sensor.
Second, the manual labor to glue a core to a board is not very efficient or cost effective and the glue curing cycle is typically relatively long. Labor and curing costs increase the overall costs associated with providing these types of Hall effect current sensors.
One other approach to mounting a Hall effect chip within a core gap has been to mount the chip on a board, position the core in a housing cavity with the circuit board mounted chip appropriately juxtaposed within the gap, fill the cavity with epoxy potting compound and bake the filled housing for several hours to completely cure the epoxy. As in the case of the glued donut shaped core, the manual labor required to pot the core and board is relatively expensive. Moreover, the baking time required to cure the epoxy reduces manufacturing throughput. Furthermore, the requirement for a housing increases parts count and hence overall configuration costs.
Yet one other approach to mounting a Hall effect chip within a core gap has been to mount a circuit board within a bobbin and mount a Hall effect chip to the circuit board where right angle pin connectors from the chip protrude out of apertures in the bobbin for connection to one or more other circuit boards. A core lamination stack is inserted into the bobbin with the bobbin formed to arrange the core and chip with respect to each other such that the chip is within the gap. Thereafter, the bobbin, core, chip and board are inserted into a first piece of a housing with the pin connectors protruding out housing apertures and a second housing piece is snapped together with the first piece to secure all of the components inside. The housed configuration forms a complete Hall effect current sensor.
This solution, unfortunately, requires a relatively large number of components and therefore increases costs appreciably. In addition, the pin connectors used with this type of assembly are relatively flimsy and have been known to break when used in typical industrial environments. Moreover, the pin connectors are often bent prior to installation or may be located imperfectly and therefore make installation relatively difficult. Furthermore, if the laminations are not clamped tightly by the housing, the laminations may shift laterally or rotate within the housing due to shock or vibrations. Such shifting and rotation will often result in changing the size of the core gap which alters the sensitivity of the sensor configuration.
One constraint on core size is the required dimensions of the conductor that passes through the core. To this end, conductors are typically selected based on the expected maximum steady state current passing through the conductors to ensure that heat generated by I2R losses or eddy currents does not cause the conductor temperature to exceed maximum limits defined by UL or IEC specifications. Heat generated by conductor I2R losses varies inversely with conductor cross-sectional area and with the square of current. Therefore, if conductor temperature is to be maintained, doubling the current requires a conductor with four times the original cross-sectional area.
In addition to current considerations, one other factor that may dictate conductor characteristics is the type of application in which the conductor is employed. For instance, in some applications a conductor may form a bus bar where ends of the bus bar have to have certain dimensions in order to facilitate hookup of other components via common size terminal lugs and mounting hardware that conforms with industry standards.
In some soft motor control (SMC) modular applications (e.g., high amp power pole sub-assemblies), bus bars are designed to minimize I2R have the largest area possible to facilitate maximum heat dissipation. For instance, where a module footprint is twelve inches by two inches, the bus bar may be designed to be thirteen inches long and two inches wide, the additional inch in length provided so that the bar extends from a module housing for linking to other system components. In such a case the core of a hall effect current sensor must have dimensions that can accommodate the required bus bar width. Thus, in the case of a torroidal core the core diameter would have to be greater than two inches to accommodate the bus bar therein.
Unfortunately, in the example above where the module footprint is twelve by two inches, if a core is provided about the bus bar, the core will exceed the module footprint. For instance, assume a core having side or annular members that are xc2xc inch thick. In this case, the core about the bus bar would exceed the footprint by xc2xd inch along the width dimension (i.e., xc2xc inch on either side of the width). A couple of ways to deal with this problem would be to increase the module footprint, reduce current levels in the bus bars or change the bar cross-section to square versus rectangular. Unfortunately all of these options severely compromise product size, ease of using standard termination lugs or mounting hardware and limit maximum optimal current levels.
A commonly owned patent application filed on even date herewith which is entitled xe2x80x9cSnap Fit Hall Effect Circuit Mount Apparatus and Methodxe2x80x9d teaches one assembly that addresses many of the problems discusses above. To this end, the snap fit concepts in this reference teach a sensor mounted to a resilient clip member where the clip member is securely mountable within a core sensing gap such that the sensor is positioned substantially within the sensing gap. Thus, this solution addresses the problems discussed above with respect to mounting a sensor within a core gap by providing an inexpensive, low-parts count and simple to manufacture and configure assembly. Unfortunately, this solution does not address the other problems discussed above and related to accommodating a bus bar width and core in a relatively small area (e.g., within a small width footprint).
Therefore, it would be advantageous to have a simple and inexpensive solution for accommodating a bus bar width and core in a small width footprint without reducing current ratings for the bar or increasing size of the product.
It has been recognized that a bus bar can be notched along a relatively short segment thereof and on either side such that the notched section and members of a core there around together are within a maximum dimension corresponding to a configuration footprint. In this manner, the wider portions of the bus bar operate as a heat sink for the notched segment and other bar segments and current rating is relatively unaffected. It has also been recognized that by configuring the core so as to have specific dimensions relative to the bus bar the overall size of the bar and core can be minimized and a simple method is facilitated for positioning the core and bar with respect to each other. To this end, generally, the core is formed such that a gap formed thereby is wider than the notched portion of the bus bar and a core space defined by an internal surface of the core is sized to receive the notched core segment so that the notched segment can be manipulated through the gap and into the core space.
Consistent with the above description, the present invention includes an apparatus for passing current and sensing magnetic field flux formed by the current, the apparatus comprising a current conductor having a length that extends between first and second conductor ends and first and second edges that extend substantially along the entire conductor length, the edges forming first and second notches at a central conductor segment such that the central segment has a central width dimension that is less than the width dimensions of conductor segments adjacent thereto and a rigid magnetically permeable guide core extending between facing first and second core ends and an internal surface formed about a core space, the first and second core ends defining a sensing gap having a gap dimension therebetween that is greater than the conductor thickness and less than the conductor width, the core including first and second member segments on opposite sides of the core space, wherein the central conductor segment is moveable through the gap and substantially into the core space with the conductor width aligned substantially perpendicular to the gap dimension and the core space is configured to allow the conductor, once substantially in the core space, to be moved into an operating position with the second member segment passing at least partially through the second notch and one of the first member segment and the gap passing at least partially through the first notch.
The invention also includes a method for passing current and sensing magnetic field flux formed by the current, the method comprising the steps of providing a current conductor having a length that extends between first and second conductor ends and first and second edges that extend substantially along the entire conductor length, the edges forming first and second notches at a central conductor segment such that the central segment has a central width dimension that is less than the width dimensions of conductor segments adjacent thereto, providing a rigid magnetically permeable guide core extending between facing first and second core ends and an internal surface formed about a core space, the first and second core ends defining a sensing gap having a gap dimension therebetween that is greater than the conductor thickness and less than the conductor width, the core including first and second member segments on opposite sides of the core space, moving the central conductor segment through the gap and substantially into the core space with the conductor width aligned substantially perpendicular to the gap dimension and moving the central segment into an operating position with the second member segment passing at least partially through the second notch and one of the first member segment and the gap passing at least partially through the first notch.
These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.